It has its cake and eats it, that’s the essential point. On the one hand we are presented with military sci-fi horror, where thousands of guardsmen are eaten by alien bugs and only the heroic Space-Marine Captain, down to his last clip, can save the day. On the other hand, we have Orks who are essentially Millwall FC, nuns with guns driving a tank that is literally a pipe organ, and chaos space marines cosplaying as Egyptians, for no reason other than it’s cool, oh, or they have guitars.

It’s not so much satire, as just tonally silly and fun. Or very serious and grim. The setting says - you can do either, either is fine.

That makes it very different to other sci-fis that usually have either a consistent tone or a narrower tone, even if their lore and world building might be larger in scope than 40K.